PHYSICS   LECTURE   DEMONSTRATIONS

M-E2a : Loop-the-Loop (Large)

UNIT: Mechanical energy. Kinetic energy of rolling.
PURPOSE: Illustrate work-kinetic energy theorem. 
EQUIPMENT:
1. Loop-the-loop track on a stand (there are two, larger is better; in both cases balls with good traction must be chosen so as to avoid sliding friction)
2. Several balls to roll
SUGGESTIONS: 
1. Release a ball from a low position on the longer side of the loop, and note that it fails to complete the run.
2. Release a ball from near the top of the longer side, and note that it easily makes the run.
3. Ask what is the minimum release height. Try.
4. Try dropping from h=2.5R (R=radius of the loop)
5. Try dropping from h=2.7R, then a bit above that height to compensate for rolling friction.
DISCUSSION: Prediction of h min based on translational KE alone of a ball gives h=2.5R, independent of mass and r, the radius of the ball (assuming r<<R). If rotational KE is included, prediction becomes h=2.7R. For the ball to make the loop run, one must compensate for the rolling friction. To obtain R accurately it is best to measure the diameter 2R, taking into account the position of the ball's center relative to the track. See attached.
Instructions-1

Instructions-2

Instructions-3

Instructions_4
 
 

1.                  2.                3.

4.                                 5.

1. Measuring the diameter 2r of the loop-the-loop (using two balls, from center to center)
2. Finding the release point at h=2r
3. The ball fails to complete the loop
4. Finding the release point at h=2.7r+d, d approx. 3cm to compensate for the rolling friction
5. The ball at the top of the loop on its way to complete the run